Foxes don't just interact with other animals; they can also have an impact on the various plants with which they share their territory. While plant material (particularly fruit) is eaten by foxes, their influence extends far beyond this. The construction of earths (dens) and their activity in the vicinity also alters the plant communities – plants are buried, trampled or destroyed by playing cubs, while fox droppings and decaying prey remains act as fertiliser, changing the chemistry of the soil and promoting the growth of some species. Generally, the plant community is richer (i.e. has more species) in the vicinity of fox earths than in surrounding undisturbed land.
Foxes exert what we call a bioperturbation effect; in other words, they mix up soil layers when they dig their earths, bringing up soil richer in minerals and nutrients and higher in pH (i.e. less acidic) than the soil on the surface. Bioperturbation can produce small fertile patches of soil in forests that can enable herbaceous species to gain a foot-hold. In addition, the fox’s penchant for fruit may help plants spread and colonise new areas by dispersing the seeds in their droppings: a process known as endozoochory.
Sowing the seeds
In a paper to the journal Restoration Ecology during 2010, Luis Matias and colleagues at the Universidad de Granada in Spain reported that foxes living in south-east Spain's Sierra Nevada National Park were efficient seed dispersers for a wide range of woody plant species. The seeds of the thornless blackberry (Rubus ulmifolius) and fig (Ficus carica) were most commonly found in fox scats and, in total, Matias and his team recovered almost 11,000 seeds from 137 fox scats that they collected over three years (2004-2006); neither the pine marten (Martes martes) nor wild boar (Sus scrofa) had more seeds in their droppings. Moreover, the biologists found that passage through the fox digestive system didn't affect the viability of the seeds.
Similarly, working in the León province of north-west Spain, Ángel Hernández at the University of Valladolid found that almost 80% of the fox droppings he analysed contained cherry seeds, an average of 37.5 per dropping, all of which were intact. In a 2008 paper to the Polish Journal of Ecology, Hernández suggested foxes may be an important disperser for cherries dislodged by birds or by the wind that might otherwise be lost.
In a paper to the journal Weed Research, published in 1976, Han Brunner and colleagues at the Vermin and Noxious Weeds Destruction Board in Australia (now the Department of Environment and Primary Industries) reported on the number and viability of blackberry seeds recovered from the scats of foxes from two populations in the south-east of the country; one in Dartmouth and the other at Sherbrooke. Brunner and his co-workers collected 1,665 droppings and found that during the late summer and early autumn 80-90% of the Dartmouth and 40-55% of the Sherbrooke scats contained blackberry seeds, most containing several hundred per dropping; the greatest haul from a single scat was 950 seeds.
Brunner and his colleagues knew that blackberry seeds have a tough, multi-layered coating (called a pericarp) that protects the kernal inside from damage and, to test whether the seeds were still viable, they planted some and watched to see whether they germinated. The biologists reported that 22% of the seeds from the Dartmouth fox scats and 35% of the Sherbrooke seeds subsequently germinated. Unfortunately, Brunner and his team don't say how many seeds from each population they incubated, but they do note that a total of 6,000 seeds were planted. If we assume an even split between the populations (i.e. 3,000 from Dartmouth and 3,000 from Sherbrooke) then, overall, just fewer than one-in-three (28.5%) of the seeds germinated.
This may make it sound like passage through a fox's intestine is detrimental for a seed, but blackberries have a notoriously low germination rate and only about 30% of the seeds collected directly from berries, which the researchers incubated with the ones recovered from the fox scats, germinated. Overall, Brunner and his colleagues concluded that:
“[viable] blackberry seed is dispersed extensively by foxes and birds in parts of the Victorian bushland”.
Similarly, in a 1998 paper to Plant Protection Quarterly, Paul Meek reported that half of the bitou bush (Chrysanthemoides monilifera) seeds, an invasive 'weed' species introduced to Australia from South Africa in 1852, recovered from fox droppings collected on Bherwerre Peninsula at Jervis Bay in new South Wales germinated successfully.
Italian biologists Giovanna Aronne and Danilo Russo found that blackberry seeds in fox scat actually had an increased “germinability” compared to those taken directly from the fruit. In their 1997 paper to Plant Biosystems, Aronne and Russo note:
“At Capo d'Orso, 8 days after sowing 58% of seeds from scats had germinated compared with 3% of those from fruits.”
Anyone who has inspected (particularly blackberry) seed-laden scat will have noticed that it is decidedly brittle and fragile owing to the high percentage of undigested pericarps. Breakage subsequently results in seeds being scattered in the vicinity of the droppings and this probably enhances dispersal potential.
These studies infer that foxes are potentially good dispersers of various plant species, but real world examples have been limited. Recently, however, we have also gained an insight into how plant dispersal can suffer in the absence of foxes. In 2016 a team of Spanish biologists published a paper in the journal Biological Conservation suggesting that foxes play a significant role in the dispersal of the lotus tree, Ziziphus lotus. Lotus trees are distributed across North Africa, but in Europe are found only in Spain and Sicily, where they're considered a keystone species, providing shade/shelter, enhancing the growth of several other plant species and creating islands of soil fertility in otherwise highly arid (dry) environments.
Between 2012 and 2014 the researchers collected 590 fox scats from the Iberian Peninsula and, as with previous studies, found that travelling through a fox's gut improved the germination potential – all seeds collected from the fox scats germinated, while about one-fifth collected from the trees themselves failed to germinate. Moreover, they found that although fox activity didn't vary with habitat cover, the number of lotus fruits they ate did – much more lotus fruit was eaten in areas with more than 60% vegetation cover. It appears that habitat loss, even if it doesn't directly mean the removal of lotus trees, can cause a collapse in their seed dispersal because foxes are less likely to eat their fruit in fragmented landscapes than in continuous natural ones. The biologists concluded:
“Dispersal service and natural regeneration in many Ziziphus habitat remnants will possibly cease in the future if habitat loss continues.”
Hitching a ride
Finally, there is one other animal that foxes may help spread that warrants a mention: the humble slug. In a fascinating 1987 paper to the Journal of Conchology, the late mollusc biologist Stella Davies suggested that, in urban areas, foxes may be an important means of transport for certain slug species – the slimy travellers hitching a ride on fox fur coats. Davies noted that slugs have been found attached to the tails of domestic cats and are probably capable of latching on to the fur of most mammals allowing them to be carried to new locations. In this case, she was referring specifically to the Portuguese slug (Arion lusitanicus) in Britain, and noted:
“...foxes probably offer the best means of transport between habitats for lusitanicus in urban areas. Increases in urban foxes may have had an important influence on the distribution of lusitanicaus in Britain.”
Presumably many slugs, as well as small insects and parasites, could also find new habitats in a similar way.